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In this analysis it is shown that for the reference mission, with a crew of 3 supported for 14 days, 108.8 kg of consumables are required for life support maintainence. This does not include margins, the initial cabin atmosphere, initial water supplies, or life support before LEO departure. This is primarily food, oxygen and hydrogen. However, if a lunar oxygen extraction facility was sent with an earlier telerobot landing, only 34.9 kg of consumables would be needed, as 66.9 kg of the above 108.8 kg is oxygen.
With an open-loop system, life support maintainence supplies would mass 650.2 kg for the reference mission. This assumes dirty clothes are stowed, rather then washed, as washing them in-situ would raise consumables required to 1175 kg.
Issues resolved in this paper involve the crew's needs for:
Details not included in this essay are: using pyrolosis apparatus to recycle organic wastes, and plant life recycling. These would both increase efficiency of the life support loop dramatically.
When we're keeping crews on the moon for any length of time, comsumables will become a major issue. One year of consumables for 4 crewmembers would total over 40.9 tons if we were to use a conventional open-loop life support system. That would require over 185 tons of mass (mostly fuel) in LEO to deliver the consumables to the moon, which is clearly out of the question. Larger crews and permanent manning of the outpost only worsens the problem.
Table 1: Human Consumable Consumption (per person)
kg per day
Air 1.55
Oxygen 0.84
CO2 removal 0.71
Food 1.77
Food Solids 0.62
Water in food 1.15
Potable Water 2.38
Food prep water 0.76
Drink 1.62
Sanitary Water 9.27
Dish wash water 2.45
Hand & face wash water 4.09
Shower water 2.73
Water 12.99
Urine flush water 0.49
Clothes wash water 12.50
--------
27.98 kg.person.day
[Designing for Human Presence in Space: An Introduction to
Environmental Control and Life Support Systems, NASA RP-1324]
To cut the consumable mass requires extracting consumables from the moon, or reusing materials already shipped to the moon.
The moon's resources include little needed for life, except oxygen, which is the most plentiful element on the moon's crust. Oxygen is the major element in water, and is needed for breathing. Assuming water can be assembled to produce nightime power in fuel cells from hydrogen and lunar-extracted oxygen, only the H2 needs to be imported for the water requirements, cutting the mass down to 11% of the original H2O. Revising the table to account for lunar oxygen use gives us these figures:
Table 2: Mass for Import with Lunar Oxygen Extraction
kg per day
Air 0.71
Oxygen 0
CO2 removal 0.71
Food 1.77
Potable Water 0.26
Sanitary Water 1.03
Water 1.44
--------
5.90 kg.person.day
Lunar Oxygen Extracted 22.75
Hydrogen Imported 2.73
Food Imported 1.77
CO2 Scrubber Imported 0.71
This brings down the mass of consumables for import for 1 year with 4 crewmembers to 8.6 tons from 41 tons without use of lunar oxygen, or to 21% of the previous value. While this is still a large mass to be imported regularly, it is now in the realm of feasible endeavors. However, extraction of the required 33.2 tons of lunar oxygen is difficult and energy-intensive, and it is possible to recycle much of the above mass.
The human body loses much of its water through exhaling and perspiration into the atmosphere. This water must be condensed out of the cabin air before it gets uncomfortably humid, and water extracted in this manner is very pure. With some relatively lightweight membrane filters, it is possible to create potable water out of the humidity control atmospheric condensors, needed anyway for operation in an enclosed environment, to supply much of the crew's drinking water.
An average human exhales 2.28 kg of water each day. The daily requirement for pure water is 2.38 kg, leaving 100 g daily to be imported. Assuming lunar oxygen is used, 11 g of H2 must be imported per person/day for drinking water.
It is also very easy to filter sanitary water (for showers, washing hands, etc.) for reuse. Exposing it to raw sunlight through a pane of UV-transparant quartz, which also boils the water, would kill nearly all microbes, putting the concentration far below levels for safety. If passed through a series of filters, nearly all of the sanitary water could be recovered. Assuming 99% recovery, and using lunar oxygen for the oxygen component of replacement water, 99.89% of sanitary water imports are eliminated.
Table 3: Mass for Import with Water Recycling
kg per day
Air 1.55
Oxygen 0.84
CO2 removal 0.71
Food 1.77
Potable Water 0.011
Sanitary Water 0.010
Water 0.504
Urine flush water 0.49
Clothes wash water 0.014
--------
3.85 kg.person.day
Lunar Oxygen Extracted 0.282
Hydrogen Imported 0.035
CO2 Scrubber Imported 0.71
This would now be 5.6 tons for a crew of 4 staying 1 year, including 1.2 tons of oxygen. If this breathing oxygen was extracted from the lunar regolith, only 4.4 tons of imports would be required, 10.8% the original 40.9 tons. 35.2 tons of imports are saved by recycling water.
Food contians, on average, about 65% water. Food can thus be shipped from Earth in a dehydrated form and water could be added on serving. The food imported would then only weigh 35% of the original 1.77 kg per person-day, plus the hydrogen component of the water to be added (11% of 1.15, or 0.12 kg), making for only 0.74 kg of imported food, 41% of the original 1.77 kg. This also reduces volume requirements for shipping and is widely practiced on shuttle missions.
Table 4: Mass for Import with Dehydrated Food
kg per day
Air 1.55
Oxygen 0.84
CO2 removal 0.71
Food 0.74
Food Solids 0.62
Water in food 0.12
Potable Water 2.38
Sanitary Water 9.27
Water 12.99
--------
26.96 kg.person.day
Lunar Oxygen Extracted 1.03
Hydrogen Imported 0.12
Food Imported 0.62
CO2 Scrubber Imported 0.71
Removing the oxygen from the water portion of imported food saves 1.5 tons of imports.
The most obvious byproduct from humans is CO2. Usually, on short, open-loop space missions such as the Shuttle, LiOH is used for CO2 removal. 1.4 kg of CO2 is removed for every kilogram of LiOH, and the body produces 1.00 kg of CO2 daily. This means 714 grams per person/day would be needed, or 1.0 tons of LiOH for a crew of 4 staying a year. CO2 also contains moon-scarce carbon, which is a valuable resource for a lunar outpost farther down the road. Extracting CO2 from the cabin atmosphere with throwaway chemical reactions clearly is unacceptable for a long-term stay.
Fortunately, CO2 can be removed and recycled with marginal difficulty through simple chemical reactions into oxygen for breathing and carbon for assimilation in plant life. This is a two-step process to recover the oxygen, starting with the Sabatier reaction:
Figure 5: Sabatier Reaction
CO2 + 4H2 = CH4 + 2H2O
We are then left with two by-products: methane (CH4) and water (H2O). These can be further processed as folllows:
Figure 6: Processing Sabatier Products
Electrolysis: 2H2O = 2H2 + O2
Pyrolysis: CH4 = C + 2H2
This reaction requires hydrogen, which can be borrowed from fuel cells during the day when the photovoltaics are active and there is a surplus of energy for the outpost. The hydrogen used in the reaction would be recycled with very high efficiency. Assuming 99.6% efficiency, for every 1000 kg of oxygen recycled, 1 kg of replacement hydrogen will be required.
Carbon released in this reaction, in the form of charcoal, can be used to extract volatiles and contaminants from the cabin atmosphere. Without constant filtering by activated charcoal, the habitat would accumulate smells very quickly and soon begin to reduce productivity, thus the need to import filters is eliminated by this useful by-product. After the charcoal had absorbed to its saturation point, it could be pyrolised into water, carbon dioxide, nitrous oxide, and a host of oxides of other elements, each of which could be stored or recycled. Alternatively, the spent charcoal could be assimilated into plant life (probably after some chemical processing).
The oxygen could be recovered with a similar high efficiency, likely over 99%. This makes 8.5 grams a day of lost oxygen and hydrogen, compared to 714 grams of conventional LiOH scrubber needed to absorb the same amount of CO2. That not recovered would be made up with Lunar oxygen extracted from regolith. This gives us the following table:
Table 8: Mass for Import with Oxygen Recycling
kg per day
Air 0.00072
Oxygen 0
CO2 removal 0.00072
Food 1.77
Potable Water 2.38
Sanitary Water 9.27
Water 12.99
--------
26.42 kg.person.day
Lunar Oxygen Extracted 0.0084
Hydrogen Imported 0.00072
CO2 Scrubber Imported 0
Recycling the carbon dioxide reduces weight for import by 1.2 tons, plus basically eliminating the need to import LiOH canisters, which add up to 1.0 tons a year. Regenerative oxygen recycling thus saves 2.2 tons of imports per year.
Combining all these factors for recycling consumables reduces nearly all imports except for hydrogen and food. Later on, food can be grown on the moon in greenhouses, but the mass investment takes so long to amortize with a small crew that this is not a near-term solution.
Table 9: Total Import Required
kg per day
Air 0.00072
Oxygen 0
CO2 Scrubber 0.00072
Food 0.74
Food Solids 0.62
Water in food 0.12
Potable Water 0.011
Sanitary Water 0.010
Water 0.068
Urine flush water 0.054
Clothes wash water 0.014
--------
0.83 kg.person.day
Lunar Oxygen Extracted 1.7560
Hydrogen Imported 0.21036
Food Imported 0.62
Back to the example of a 4-person crew's requirements for a year, if a number of relatively easy-to-implement steps are taken to reduce the consumables, only 1226 kg of mass needs to be imported from Earth annually compared to 40.9 tons needed for an open-loop system, 3.0% of the original value.
This reduced value is certainly feasible, but further mass reduction is possible if excreta and water filterings are used to grow plants. Greenhouses would be massive, but if made from lunar glass, ceramics and metals, they could possibly be used economically. One complication is that all of the biomass would need to be imported from Earth as hydrogen for water, atmospheric nitrogen, and food, which is then converted into fertilizer for plants to assimilate into biomass. The biomass would otherwise accumulate to the tune of 0.84 kg each person/day, however, so dedicated imports won't be needed for the short term. Nonrecyclables such as urine, fecal matter, as well as sweat solids and other filterings from the water recycling system, will simply be stored and accumulated awaiting later use as plant fertilizer.
With food grown in-situ, the life support loop would be nearly closed. Even assuming no usable hydrogen is reclaimed from the waste by the plants, the import requirements would drop from 840 grams per person/day to 21 grams per person/day, with hydrogen the only material being imported (aside from lunar oxygen extraction). This hydrogen import is inevitable unless lunar ice deposits are found or sizeable quantities are produced as a byproduct of processing regolith for Helium-3.
With plants producing all of the crew's food, the life support loop would be 99.925% closed, with the case of four people to support for a year would require 30.7 kg of descent engine fuel residuals, as opposed to 40,850 kg of imports from Earth. This is certainly within the reach of technology from even 50 or more years ago.
Clearly, the seemingly-insurmountable consumble problem can be dealt with like any other challenge facing a permanent lunar outpost.
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